American Mineralogist, Volume 73, pages 261-273, 1988

Pyroxene exsolution: An indicator of high-pressure igneous crystallization ofpyroxene-bearing qv rtz syenite gneiss from the High Peaks region of the

Adirondack Mountains

Plur- W. Or,r,rr..q.Department of Geology and Geography, Vassar College, Poughkeepsie, New York 12601, U.S.A.

How.q.nr W. Jlrrn, Er.rzlnnrs B. J.q.rrnDepartment of Geology and Geography, University of Massachusetts, Amherst, Massachusetts 01003, U.S-A-

Ansrn-q.cr

Relict, coarse, high-temperature inverted pigeonite lamellae in host augite and augitelamellae in host inverted pigeonite are locally preserved in metamorphosed igneous rocksfrom the Adirondack Mountains of New York State. The exsolution history of thesepyroxenes consists of (l) crystallization of high-temperature augite or pigeonite, (2) exso-lution of coarse "00l" lamellae of augite in host pigeonite and pigeonite in host augite,(3) fautting of lamellae and then of host along (100) as lattice parameters change duringcooling, (4) inversion of pigeonite to orthopyroxene, and (5) decomposition of the lamellaeto produce intergrowths ofaugite and orthopyroxene along (100) ofhost grain.

These textures have been observed in pyroxenes that span the full range ofcompositionsfound in Adirondack anorthositic and syenitic rocks. An inverted pigeonite from a pyrox-

ene-bearing quartz syenite gneiss collected in the Mount Marcy quadrangle yields a rein-tegrated composition of Wo,r,EnorFsro, indicating that igneous crystallization of this rocktook place at pressures greater than or equal to approximately 9 kbar.

INrnorucrroN

This paper describes pyroxene exsolution textures foundin metamorphosed igneous rocks from the AdirondackMountains of New York State. These exsolution texturesare petrologically significant in that they allow one to "seethrough" the effects of regional metamorphism and tostudy the igneous history of the rocks that contain them.Pyroxenes described in this report were found in rocksthat range in composition from gabbroic anorthosite topyroxene-bearing quartz syenite gneiss and were collectedduring our geologic mapping of the Mount Marcy andSantanoni quadrangles in the High Peaks region of theAdirondacks. Sample locations are shown on Figure 1,and sample descriptions are given in Appendix 1.

The Marcy anorthosite massif crops out in a roughlyheart-shaped pattern and covers approximately 3000 km'z(Fig. l). It has generally been charucterized as consistingof a core of anorthosite surrounded by gabbroic anor-thosite (Buddington, 1939,1969; Davis, 1971). Mappingby Ollila (1984), however, has shown that there are sig-nificant amounts of gabbroic anorthosite within the coreregions of the massif. Core rocks are characterized byigneous textures. Subophitic gabbroic anorthosites andgabbros in which garnet coronas around mafic mineralsare the only evidence of regional metamorphism are rel-atively common. Gabbroic anorthosite gneiss, character-ized by plagioclase augen in a granoblastic matrix of pla-

0003-004x/88/0304-026 I $02.00

gioclase, pyroxene, hornblende, and garnet, occurs at themargins of the massif and in localized shear zones withinthe core of the massif. Syenite gneisses typically overliethe gabbroic anorthosite gneiss found at the margins ofthe anorthosite massif.

Valley and O'Neil (1982) and Valley (1985) presentedevidence that anorthosite in the Adirondacks was intrud-ed at relatively shallow levels (<10 km) and then laterunderwent granulite-facies metamorphism. This was asignificant finding in that it was the first actual evidencethat Adirondack anorthosite was not intruded underpressure-temperature conditions similar to those that oc-curred during regional metamorphism. This finding gavesuppolt to the suggestion, made on the basis of analogywith Labrador anorthosite massifs, that anorthosite in theAdirondacks may have intruded under anorogenic ratherthan synorogenic conditions (Berg, 1977; Emslie, 1978).Evidence presented in this paper suggests, however, thatpyroxene-bearing quartz syenites closely associated withand commonly thought to be coeval with the Adirondackanorthosite were intruded at depths greater than l0 kmand perhaps as great as 30 km. The most important evi-dence leading to this conclusion is igneous exsolution tex-tures in Fe-rich pyroxenes from quartz syenites. Low-Capyroxenes that exhibit these textures are sufrciently Fe-rich that they only could have crystallized at higft pres-sures. In order to develop this argument, it is first nec-essary to briefly describe exsolution in pyroxenes.

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262 OLLILA ET AL.: PYROXENE EXSOLUTION

GRAN|TtC, SYENtTtC,8 MIXED GNEISSES

Mount Marcy anorthosite massif. Locations for samples 9-17 , AL-4, BA-5, GB-2, and

4tP.t5'z4030'

l-vT-TlANORTHOSITE

Fig. 1. Generalized geologic map of thePO-17 are indicated on the map.

t - - - ll + + l

CALC - SILICATE ROCKS

PynoxnNn ExsoLUTroN

Optimal phase boundary theory was used by Robinsonetal. (1971, 1977), Robinson(1980), andJaffeeral. (1975)to explain why pigeonite lamellae in an augite host andarrgite lamellae in a pigeonite host are oriented along non-rational planes. The theory states that monoclinic pyrox-enes with identical D dimensions and differing a, c, andB will have two planes of exact dimensional fit. One plane,"001," is close to (001), its orientation being controlledby the diference between the c dimensions of the hostand lamellae pyroxenes. Another plane, "100," lies closeto (100) and is controlled by the difference between the cdimensions of the pyroxenes. Since the lattice parameters

are functions of pressure, temperature, and chemicalcomposition, the orientation of the best-fit planes is alsoa function ofthese variables. The rapid expansion ofpi-geonite as it goes through the P2,/c = C2/c reversible,nonquenchable transition is of particular importance ininterpreting exsolution temperatures. At temperaturesabove the stability of P2,/ c pigeonite, a and c ofpigeoniteare greater than a and c of augite. At temperatures whereP2,/c pigeonite is stable, the reverse is true. Because ofthese relationships, high-temperature lamellae of pigeon-ite in host augite or augite lamellae in host pigeonite ori-ent in obtuse angle p [c rr "001" < c A (001)]; whereaslow-temperature lamellae orient in acute angle 0 Fig.2).High-temperature lamellae are much coarser (approxi-

OLLILA ET AL.: PYROXENE EXSOLUTION 263

mately 10-100 pm thick) and spaced farther apart thanlamellae formed at lower temperatures, which are lessthan I pm thick. Examples of high-temperature exsolu-tion in low-Ca pyroxenes (inverted pigeonite) are rela-tively common in plutonic igneous rocks and have beendescribed from many localities. This type of exsolutionwas originally described by Hess (1941, 1960) and hasbeen described in Adirondack rocks by Davis (1971),Ashwal (1982), and Jaffe et al. (1983). Robinson et al.( I 977) described high-temperature ( - 1000 'C) exsolutionin host augites from the Bushveld complex in South Af-rica and the Nain complex in Labrador, and more re-cently Livi (1987) has described similar exsolution fea-tures in pyroxenes from the Laramie anorthosite massif.Before the work of Ollila et al. (1983, 1984), this type ofexsolution had not been recognized in the Adirondacks.

In contrast to the exsolution textures visible in mono-clinic pyroxenes, exsolution between orthorhombic andmonoclinic pyroxenes is much simpler. The only planethat is at all similar between orthopyroxene and clino-pyroxene is (100), and lamellae of clinopyroxene in or-thopyroxene host or orthopyroxene in clinopyroxene hostorient along this plane. As pointed out by Robinson(1980), this type of exsolution is common in metamor-phic pyroxenes and in magnesian igneous pyroxenes thatcrystallized at temperatures below the stability of pigeon-ite. This type of exsolution also occurs during the latestages of exsolution of igneous pyroxenes.

Exsolution textures in Adirondack rocks are compli-cated by the fact that igneous rocks have been affected bya regional metamorphism. This metamorphism has re-sulted in complex exsolution textures that have beenpoorly understood. In a few locations, however, igneousrocks were only slightly deformed during regional meta-morphism, and pyroxenes in rocks from these localitiesretain recognizable high-temperature exsolution texturesthat have proved to be the key in unraveling the exsolu-tion history of Adirondack pyroxenes.

Figure 2 is a schematic diagram showing a model forthe exsolution history of metamorphosed igneous augite.Stages I through 3 are based on descriptions ofunmeta-morphosed igneous pyroxenes (Robinson et al., 1977;Robinson, 1980). Stages 4 through 6 are based on obser-vations of pyroxenes from Adirondack rocks. The dia-gram is applicable to both augite and pigeonite hosts, butinverted pigeonite that retains coarse "001" augite la-mellae is much more common than host augite with rel-ict, high-temperature "001" inverted pigeonite lamellae.Figures 3 through 5 are photomicrographs ofgrains thatillustrate the exsolution processes outlined in Figure 2.

Relict, high-temperature exsolution lamellae in hostaugite were found initially in a coarse-grained gabbroicanorthosite from the central part ofthe Santanoni quad-rangle (sample 9-17, Fig. 3). This rock is subophitic withboth augite and inverted pigeonite interstitial to plagio-clase. Subsequently, similar exsolution textures were foundin rocks from the central part of the Mount Marcy quad-rangle (GB-2 and BA-5, Figs. 4 and 5). These rocks are

Fig. 2. L .oa.r based on ol."*u,roo, o, orro*Ln" texturesfor the exsolution history ofigneous augite in the Adirondacks.The sequence is (l) crystallization of homogeneous augite; (2)exsolution of coarse "001" pigeonite lamellae, which orient inobtuse angle B of the host augite; (3) faulting of pigeonite la-mellae during cooling (Robinson etal., 1977); (4) faulting of thehost augite along (100), inversion ofpigeonite and decomposi-tion of"001" inverted pigeonite lamellae along (100) ofthe hostaugite; (5) formation of pyroxene "mesoperthite"-all the ortho-pyroxene that originally was in "001" lamellae now forms (100)lamellae in host augite; (6) orthopyroxene lamellae coarsen, andfine pigeonite lamellae, which orient in acute angle B of hostaugite, exsolve from host augite. Note that low-temperature (100)orthopyroxene lamellae may exsolve so as to protrude beyondthe confines ofthe augite host.

weakly foliated and consist of medium- to fine-grainedgranoblastic aggregates of garnet, pyroxene, and feldspar,but also contain coarser (>2 mm) pyroxene grains thatretain relict igneous exsolution textures. Augite fromsample 9-17 shows textures equivalent to stage 4 in Fig-ure 2. Augite in samples BA-5 and GB-2 show texturesintermediate between stages 4 and 5 of Figure 2.It shouldbe emphasized that these sorts ofexsolution textures arequite rare. Medium- to coarse-grained augite in meta-morphosed igneous rocks from the Adirondacks mostcommonly shows exsolution textures corresponding tostage 6 of Figure 2. Augite in gabbroic anorthosite gneisstypically occurs in granoblastic aggregates and only con-tains fine, low-temperature "001" and "100" pigeonitelamellae such as illustrated between the coarser lamellaein part 6 ofFigure 2.

Inverted pigeonite containing coarse "001" augiteexsolution lamellae is common in Adirondack anortho-sitic rocks (Davis, 1971; Ashwal, 1982; Jafe et al., 1983).Such is not the case for syenitic rocks. Davis ( I 97 I ) statedthat inverted pigeonite was never observed in syenitic

264 OLLILA ET AL.: PYROXENE EXSOLUTION

Fig. 3. Photomicrograph of host augite from sample 9-17. Host augite is at extinction, and relict high-temperature pigeonitelamellae now inverted to orthopyroxene are oriented from lower left to upper right. These lamellae are faulted and have partiallybroken down to produce an intergrowth ofaugite and orthopyroxene along (100) ofhost augite (upper left to lower right). Thisgrain corresponds to stage 4 tnFig.2.

gneisses from the Saint Regis quadrangle. Bohlen and Es-sene (1978) described inverted pigeonite from Adiron-dack pyroxene-bearing quartz syenite gneiss, but the py-roxenes they described lacked the distinctive exsolutiontextures ofinverted pigeonite. Bohlen and Essene's con-clusions were based on the reintegrated compositions ofexsolved grains that contain abundant augite exsolutionlamellae oriented along (100) of host orthopyroxene. Ascan be seen in Figure 2, the assumption that these grainswere originally pigeonite is quite reasonable given theeffect that regional metamorphism has had on exsolutiontextures.

There is one potential problem, however, in interpret-ing grains that only consist of augite and orthopyroxeneintergrown along (100). Rietmeijer (1979), in an exten-sive study ofpyroxene exsolution textures in syenitic rocksfrom the South Rogaland complex in Norway, pointedout that quite similar textures could be produced by bothexsolution processes and by epitaxial overgrowths. Someof the criteria Rietmeijer used to distinguish betweenexsolution lamellae and epitaxial overgrowths includedthe width of the lamellae, the shape of the lamellae, andthe presence or absence of inclusions along hostJamellaeboundaries. These kinds of observations cannot be madeif all high-temperature lamellae have broken down toproduce augite-orthopyroxene "mesoperthite." It is un-certain how prevalent this phenomena is in Adirondackrocks, but this possibility decreases the certainty of ig-neous crystallization temperatures based on integratedcompositions of grains that do not show relict high-tem-perature-type exsolution lamellae.

Syenitic gneisses that contain pyroxenes with relict,

high-temperature exsolution lamellae were found in thenortheastern corner ofthe Santanoni quadrangle and thenorthern part of the Mount Marcy quadrangle (Fig. l).Inverted pigeonite with coarse "001" augite exsolutionlamellae was found in sample AL-4 (Fig. 6), a medium-to coarse-grained pyroxene-plagioclase-quartz-micro-perthite gneiss. Compositionally similar, but finer-grainedand more strongly foliated rocks located in the southernpart of the Santanoni quadrangle do not contain relicthigh-temperature exsolution lamellae. Exsolution tex-tures are not as well preserved in syenitic rocks from theMount Marcy quadrangle as in sample AL-4, but sampleSC-6 (Fig. 7) contains what are interpreted as epitaxialintergrowths ofpigeonite (now inverted) and augite, andboth samples PO-17 (Fig. 8) and SC-6 contain orthopy-roxene grains with abundant augite exsolution lamellaeoriented along (100) ofthe host orthopyroxene. SamplesSC-6 and PO-17 also contain host augite with relict coarse"001" lamellae.

CrrBnrrsrnv

Mineral analyses were obtained on an ETEC Autoprobeat the University of Massachusetts. Microprobe analyseswere performed at 15-kV accelerating potential and anaperture current of 0.03 prA. Standards consisted of bothsynthetic and natural silicate and oxide minerals, andcorrections were made following the procedure of Benceand Albee (1968) and using the correction factors of Al-bee and Ray (1970). Analytical results are listed in Tablel. Reintegrated analyses ofexsolved pyroxenes were ob-tained by a number of techniques. Most reintegratedanalyses were obtained by combining microprobe anal-

OLLILA ET AL.: PYROXENE EXSOLUTION 265

Fig. 4. Photomicrograph of host augite in sample GB-2. Fine(100) orthopyroxene exsolution lamellae are nearly vertical. Rel-ict high-temperature pigeonite lamellae are horizontal. This grainis intermediate between stages 4 and 5 in Fig. 2. The thick or-thopyroxene lamella in the bottom third ofthe grain is thoughtto represent an epitaxial overgrowth ofpigeonite (now inverted)on the augite grain. Note how the augite host is depleted oforthopyroxene lamellae near the coarse orthopyroxene lamella.In other grains, epitaxial overgrowths such as this one have bro-ken down to produce (100) "mesoperthite."

yses of host and lamellae with relative volumes deter-mined from point counts of photomicrographs. Approx-imately 1000 points per grain were used for this technique.In one case (BA-5), the microprobe beam was widenedto about 20 pm, and approximately 100 analyses werecollected on a number of traverses across the grain. Inthis way a significant proportion of the grain was ana-lyzed. For sample PO-17, mineral analyses of arrgite andorthopyroxene host determined by Jaffe et al. (1978) wereused in conjunction with point-count data from photo-micrographs. The assumption that augite and orthopy-roxene host and lamellae compositions are similar in thisrock was checked at Vassar College by means of energy-dispersive analyses of host and lamellae on a scanningelectron microscope equipped with an energy-dispersiveanalyzer. Use of an sEM provided advantages in that abackscattered-electron image allowed more precise posi-

tioning of the electron beam. Lamellae compositions weredetermined by comparison with host spectra. Results fromthese analyses are listed in Table 1. Optical reintegrationtechniques ofthe grain shown in Figure 8 were evaluatedby scanning crack-free areas as large as possible and ana-lyzing for Fe, Mg, Ca, and Si. Results of these analysesare shown in Figure 9. These analyses indicate that theoptical reintegration techniques provide very reasonablelimits on pigeonite compositions in this rock.

Tnrvrpnru.ruREs AND pRESSURE oF CRySTALLIZATION

Igneous crystallization temperatures for pyroxenes inanorthositic and syenitic rocks from the Adirondacks havebeen determined previously by Ashwal (1982) and Boh-len and Essene (1978). As first pointed out by Bohlen andEssene (1978), reintegrated pyroxene temperatures froma variety of metamorphosed igneous rocks from the Ad-irondacks yield temperatures well above regional meta-morphic temperatures. Bohlen et al. (1985) estimatedmaximum regional metamorphic temperatures to be ap-proximately 800 "C. Figure l0 shows integrated pyroxenecompositions from Bohlen and Essene (1978) and thisreport. Crystallization temperatures range from approxi-mately I150 "C for gabbroic anorthosite (9-17) to I100'C for metagabbro (GB-2) and 950 to 850 "C for syeniticrocks (BA-5, PO-17, P-10). Pyroxene crystallized afterplagioclase in anorthositic rocks, so liquidus tempera-tures must have been somewhat higher than the pyroxenetemperatures would indicate.

The depth at which anorthositic rocks crystallized isdifficult to determine in the Adirondacks, because region-al metamorphism has overprinted any contact aureoleassemblages that might have been useful in this respect.Martignole and Schrijver (197 1, 1973) and Martignole(1979), however, presented arguments in favor ofa deep-seated emplacement of anorthosite-charnockite suites inthe Grenville province and the Adirondacks. Martignoleand Schrijver's (1971, 1973) and Martignole's (1979) ar-guments, however, are largely based on garnet-bearingassemblages. Martignole (1979) left open the possibilitythat the garnet-bearing assemblages reflect metamorphicrecrystallization of a shallowly intruded igneous rock, butsuggested that the preservation of ophitic textures in maf-ic charnockites is most easily explained by intrusion dur-ing high-grade regional metamorphism. More recently,Martignole (1986) has presented a model that allows forboth the shallow emplacement of anorthosite and the deepemplacement of charnockitic rocks.

Valley and O'Neil (1982) concluded, on the basis ofO-isotope studies of the wollastonite deposit at Wills-boro. New York. that anorthosite was intruded at shallowlevels (< l0 km) and then was metamorphosed at higherpressures during regional metamorphism. Valley (1985)strengthened this argument with a combined phase-equi-libria and O-isotope study of Cascade slide, a monticellitemarble locality found in the Mount Marcy quadrangle.

According to shallow-intrusion models, the orthofer-rosilite origrnally described by Jaffe et al. (1978) from

266 OLLILA ET AL.: PYROXENE EXSOLUTION

Fig. 5. Coexisting augite (right) and inverted pigeonite (left) from sample BA-5. The thick "001" augite lamellae in the inverted

pigeonite and c of host augite are nearly vertical. The augite grain is intermediate between stages 4 and 5 of Fig. 2, whereas the

inverted pigeonite corresponds to stage 3. This difference in textural development is typical in these rocks. Relict igneous textures

are usually better preserved in inverted pigeonite than in host augite.

pyroxene-bearing quartz syenite gneiss in the MountMarcy quadrangle is indicative only of minimum meta-morphic pressures of approximately 8 kbar (Jaffe et al.,1978; Bohlen and Boettcher, 1981). As described above,however, re-examination of some of the specimens col-lected by Jaffe et al. (1978) has shown that Fe-rich ortho-pyroxene in these gneisses is, in fact, inverted pigeonite(Figs. 7 and 8). The composition of inverted pigeonitefrom sample PO-17, when compared to the "forbiddenzone" boundaries in the solvi oflindsley (1983), suggeststhat igneous crystallization of this rock must have takenplace at a minimum of approximately 9 kbar (Fig. 10).

DrscussroN

Valley (1985) has pointed out that there is a high degreeof uncertainty associated with the position of "forbiddenzone" boundaries in Lindsley's (1983) phase diagramswhen applied to natural pyroxenes that contain nonquad-rilateral components and has suggested that PO-17 couldhave crystallized at pressures as low as 3 kbar. It shouldbe pointed out in this regard, however, that the argumentin favor of deep crystallization for Adirondack pyroxene-bearing quartz syenites is not based solely on the com-position of sample PO-I7. Although sample PO-I7 is themost Fe-rich sample yet studied, numerous other sam-ples such as AL-4 and SC-6 show textures indicative ofigneous pigeonite and are sufrciently Fe rich to requirehigh-pressure crystallization. Samples P-10, IN-11, andSR-29 from the study of Bohlen and Essene (1978) like-wise require crystallization pressures in excess of 3 kbar.These pyroxenes contain low percentages of nonquadri-lateral components and are significantly more Fe rich thaninverted pigeonites described by Ranson (1986) from the

Nain anorthosite massif or by Frost and Lindsley (1981)

from the Laramie anorthosite massif. Samples AL-4, SC-6, PO-17, and SR-29 (Bohlen and Essene, 1978) are alsomore Fe-rich than inverted pigeonites described by Smith(1974) from a portion of the Nain anorthosite massif thatcrystallized at higher pressure (Betg, 1977) than the areadescribed by Ranson (1986). Ranson (1986) reintegratedpyroxenes in a pyroxene-bearing quartz monzonite froma part of the Nain complex that crystallized at approxi-mately 3 to 4 kbar. This rock contains the assemblageaugite + invertedpigeonite + olivine + quartz. Pyroxenecompositions from this rock (Fig. 10) are consistent withLindsley's (1983) "forbidden zone" boundaries and rep-resent the compositions at which low-Ca pyroxene is re-placed by olivine * quarrz at the pressure (3-4 kbar) andtemperature at which this pyroxene-bearing quartz mon-zonite crystallized. The relatively common occurrence ofinverted pigeonite more Fe rich than that found in sam-ple PL-187 (Fie. l0) in Adirondack syenitic rocks indi-

cates crystallizatior' al pressures well in excess of 3 kbar.If crystallization pressures for syenitic rocks are appli-

cable to anorthosite, then the conclusions presented aboveconflict with the shallow-intrusion hypothesis of Valleyand O'Neil (1982) and with the models of Whitnev (1983)

and Mclelland (1986). Although it is possible, given theuncertainties of metamorphic geothermometry and ofphase relationships for natural pyroxenes, that Fe-richpigeonite could have been produced during regionalmetamorphism, this is not thought likely. In order toevaluate this possibility, phase relations in the pyroxenequadrilateral have to be examined.

Figure 1l is a schematic representation-based on nat-ural assemblages described by Smith (197 4) and the ex-

OLLILA ET AL.: PYROXENE EXSOLUTION 267

Fig. 6. Inverted pigeonite from sample AL-4. Coarse "001"augite lamellae are oriented diagonally from lower left to upperright and c of host orthopyroxene is vertical. This grain illus-trates the pigeonite equivalent of stage 4 in Fig. 2.

perimental work of Huebner and Turnock (1980) andTurnock and Lindsley (198l)-of the phase relations be-tween olivine (ol), quartz (q), orthopyroxene (opx), andclinopyroxene (cpx) at three different temperatures (Zl <T2 < T3). As can be seen in this diagram, Ca clearlyincreases the stability of orthopyroxene over olivine forthe assemblage opx + ol + q. Ca, however, has a differenteffect on the assemblage opx + cpx + ol + q. In thisassemblage, orthopyroxene is saturated with Ca, and anyincrease in Ca content oforthopyroxene or pigeonite re-flects higher temperatures. These higher temperatures inturn favor the stability of olivine + quafiz over ortho-pyroxene or pigeonite. These effects are evident in thesolvi of Lindsley (1983) in which the "forbidden zone"for pyroxenes expands to more magnesian compositionsas the Ca content of either orthopyroxene or pigeoniteincreases. This can be seen in Figure 10, which showspyroxene compositions plotted on the l0-kbar solvus ofLindsley (1983).

The shallow-intrusion model as stated by Whitney(l 983) requires that pyroxene-bearing quartz syenite gneiss

Fig. 7. Photomicrograph of intergrown augite (top) and or-thopyroxene Oottom) from sample SC-6. The coarse augite la-mella in orthopyroxene is interpreted as an epitaxial intergrowthofaugite and pigeonite (now inverted) and suggests that pigeon-ite was the initial low-Ca pyroxene to crystallize in this rock.

be metamorphosed fayalite-rich olivine granite. If oneuses the l0-km maximum depth of intrusion suggestedby Valley and O'Neil (1982) for these rocks, then olivineshould have replaced orthopyroxene in rocks with l00Fe/(Fe + Mg) ratios of >75 (Bohlen and Boettcher, l98l)and should have replaced pigeonite with l00Fe/(Fe +Mg) ratios of > 78 at 825 "C (Lindsley and Grover, I 980).At higher temperatures, olivine should replace pigeoniteat more magnesian compositions.

According to the shallow-intrusion model, low-Ca py-roxenes with 100Fe/(Fe + Mg) ratios of >78 should re-cord metamorphic rather than igneous temperatures.Bohlen and Essene (1978) reintegrated compositions ofexsolved, Fe-rich, low-Ca pyroxenes in Adirondack sy-enitic gneisses and concluded that these rocks had ig-neous precursors that crystallized between 900 and 1000"C (Fig. l0). Any hypothesis of shallow intrusion, how-ever, requires that these rocks were originally olivine-bearing granites and would require that the temperaturesdetermined by Bohlen and Essene are metamorphic.

The requirement that metamorphic temperatures be

268 OLLILA ET AL.:PYROXENE EXSOLUTION

Tner-e 1. Electron-microprobe analyses of pyroxenes and formulae based on six oxygens

9-'t7 AL-4

Sample: Cpx H Cpx L Opx H Opx L Aug I Pig I Cpx H Cpx L Opx H OPx L Aug I Pig I Cpx H

sio,AlrosTio,CrrO.l\4gOZnOFeO.MnO

BaONaro

Total

SiAIFe3*TotalAITiCrPgs+**MgZnFe2*Mn

BaNa

Total

C U . U C

1 .900.07

50.801 9 1o.21

0.21

50.031 4 30 . 1 8

0.18

24 240.29

1 A R A

50.821 8 7o.24

0.24

50.38 s0.392 43 2.860 1 5 0 . 2 3

49.26 50.01 50.151 65 2.18 2.200.03 0 .11 0 j2

49.10 48.57o.73 0.530 09 0 .12

0.09 0.12

38 01 37 760.45 0.450.52 0.62

0.01 0.06100.40 99.97

1 .971 1.9610.029 0.025

2.0000.0000.004

0.0230.714

1 2520.0150.027

49.44 48.280.95 123ojz 0.23

0.000.12 3 .21

0.0933.84 28.010.40 0.584.49 18.79

0.016 0.0532.013 2.042

11 63 11 .21

12.47 13.050.27 0.22

21.65 21 .15

0 58 0.5399.56 99.66

1.925 1 .9230 075 0.077

2.0000 0340 004

0.0750.662

0.3230.0090.886

0 043 0.0392.036 2.026

0.072 0 074 0.0420 960 0.757 0.844

0.955 0.526 0.7930 . 0 1 9 0 0 1 2 0 0 1 40.025 0.610 0.277

0.002 0.030 0 0142.036 2.036 2019

1 7.09 16.750.20 0.18

20.40 20.79

' t 0 1 1 0 5100.44 100.61

1.957 1.9540.043 0 046

2.0000.0390 007

0.0720.511

0.4670.0060.857

0.07s 0.0782.031 2.037

0 001 0 0052 008 2.040

2.0000.0440.006

0.0620.507

0.4880.0070.842

2.0000.0530 007

0.0500.638

0.3670.0070 865

16.23 16.46 13.20 14.70

18.64 25.91o37 0 44

14.79 6.72

0.04 0.03 0.40 0.19100 78 100.00 99.70 100.43

1.937 1 926 1.925 1.9320 063 0074 0.075 0.068

2.000 2.000 2 000 2 0000 023 0 002 0.024 0.0320.002 0.001 0.003 0.003

0.68 0.21 0.68100.28 1 00.43 1 01 .09

1.958 1.968 1.9490.042 0.032 0.051

2.000 2.000 2.0000.024 0.012 0.0080.005 0 004 0.008

0.0000.058 0.029 0.0830.576 0.651 0.193

0.0030.735 1.097 0.8620.010 0.013 0 0200.569 0.191 0.81 3

31 .56 31 .400.54 0.570.39 0.60

0 9 1.3 28.648.4 49.4 42.250.8 49.2 29.3

2.0000.0050.003

0 0390.936

0.9820.0180 0 1 6

0.0032.019

0.0190.687

1 2560 015o.022

Wo.. 43.1 42 0En 38 2 36.8Fs 18.7 21.2

12.1 41.432.7 '10.7

55.1 47.9

43.7 44.8 1.1 1.4287 28.8 35.0 35.827.6 26.4 63.9 62.8

0.0522.029

zu.531 039.6

15 .843.440.8

Notei H : host, L : lamellae, | : retntegrated bulk composition. Total Fe reoorted as FeO.

"" Fe3* and Wo, En, and Fs calculated according to the method of Lindsley (1983).

t pigeonite compositions are based on opticai-reintegration of exsolved grains and host compositions published by Jatfe et al. (1978). Lamellae

cornpoiitions *ere determined by means of energy-dispeirsive analysis on a sianning electron microscope. These analyses are not as accurate as the

wavelength-dispersive analyses [ublished by Jaffe-et al. out do establish that there is no significant difference between host and lamellae compositions.

greater than 900 oC for large areas of the Adirondacksleads to a number of inconsistencies. One problem is withthe pyroxenes themselves. If metamorphic temperatureswere greater than 900 oC, pyroxenes with l00Fe/(Fe +Mg) ratios of > 92 should be hypersolvus (Lindsley, I 983).This is clearly not the case in Fe-rich rocks from theMount Marcy quadrangle (Fig. l0). A second problem isthat numerous mineral assemblages and geothermome-ters that limit metamorphic temperatures to less than ap-proximately 800 "C (Bohlen et al., 1985) are common inthe areas where a 900 'C metamorphic temperature isrequired by the shallow intrusion model. A third problemis that integated pyroxene compositions in these rocksindicate a range of temperatures. Although this is rela-tively easily explained by igneous processes, it is muchmore difficult to explain if the pyroxenes are the productof regional metamorphism. A fourth problem is that thereis no evidence of olivine pseudomorphs in any of therocks studied so far. Similar orthoferrosilite-bearing rocksfrom the Lofoten Islands of Norway retain clear-cut evi-dence that the orthoferrosilite replaced fayalite-rich oliv-ine (Ormaase4 1977). There is no textural evidence thatthis has taken place in the Adirondacks.

Another argument against a metamorphic origin forthe Fe-rich pigeonite in these rocks is the nature of theexsolution textures. Pyroxenes from similar rocks in theNain anorthosite massif (Ranson, 1981; Huntington,1980) and the Laramie massif (Frost and Lindsley, 1981;Livi, 1987) and pyroxenes from many other plutonic suites(Robinson et al., 1977) show exsolution textures similarto the relict textures observed in the Adirondacks. Thedifference between Adirondack pyroxenes and unmeta-morphosed plutonic pyroxenes is in the breakdown ofhigh-temperature pigeonite or augite lamellae (stage 3,Fig. 2) to intergrowths of augite and orthopl'roxene (stages5 and 6, Fig. 2). Although it is diftcult to assess the rel-ative importance of thermal-induced versus stress-in-duced changes, the breakdown is most easily explainedas resulting from deformation associated with regionalmetamorphism because of the correlation between rockfabric and degree of preservation of relict lamellae. Rockswith igneous textures or weak metamorphic fabrics lo-cally contain pyroxenes with relict igneous exsolutiontextures, whereas pyroxenes from granoblastic rocks onlycontain low-temperature, fine exsolution lamellae. Exso-lution textures in relatively undeformed rocks from the

OLLILA ET AL.: PYROXENE EXSOLUTION 269

TABLE 1-Continued

PO-171

Opx H Cpx H Cpx L Opx H Opx L Aug I Pig I Cpx H Cpx L Opx H Opx L Aug I Pig I Pig I

46.7 40.250.090 0 03 9 50.24

47.30u -oz0.69

0 0 099.88

1.9900.010

2 0000 0020.0020.0000.0020 2510 0081.6820.0220.031

0 0002.001

50 36 50.512.04 2.060 27 0.26

9.2s I36

15 87 16 .03o 1 7 0 . 1 8

20.64 20.11

1 1 0 1 1 399.70 99 64

1.948 1.9530.052 0.047

2.000 2 0000.041 0.0460.008 0.008

0.078 0.0710.533 0.539

0.435 0.4480.006 0.0060.855 0.833

0.083 0 0852 039 2.036

48.68 49.180.92 0.860.09 0.06

12 .06 12 49

JO.dC JO.OJ

0.50 0.470.87 0.44

o.20 0 09100.17 100.22

1.9s4 1.9650.044 0.0350 0022.000 2 0000.000 0.0050.003 0.002

49.92 49.311 . 9 1 1 . 3 10.18 0 .15

1 0 . 1 5 1 1 1 3

22.05 29.710.32 0.39'15 18 7 .47

0.68 0 51100.39 99.98

1.942 1 .9540 058 0.046

2.000 2.0000.029 0.0150.005 0.004

0 070 0.0610.589 0 657

0.648 0.9230.01 1 0.0130.633 0.317

48.69 49.720.65 0.910.06 0.0150 06 0.061.17 1 .2 ' l0 . 1 9 0 1 9

29.18 29 630.43 0.30

19.07 19.s30.000.73 0.7'l

100.65 10241

1 984 1.9900.01 6 0.010

2.000 2.0000 032 0 0330.004 0.0050 002 0.0020.029 0.0220.071 0.0720.006 0.0060.965 0.9700 015 0 .0100 833 0.830

0.057 0.0552.014 2.012

42.8 42I3.9 4.0

53.3 53.2

45.50 44.670.36 0.360 1 1 0 1 20.06 0.021.36 1 .030.37 0.41

49.81 49.921.10 1 .380.84 0.660.060.03 0.03

99.60 98.60

1.983 1 9750.017 0 .019

0.0062 000 2.0000.001 0.0000.004 0.0040.002 0 0010.009 0.0070.088 0.0680.012 0.0131.806 1.8330.041 0.0s20.039 0.0450.0010 003 0.0032.006 2.013

2.O 1 .74.6 3.5

93.4 94.8

47.86 45.97 46 180.83 0.45 0.500.13 0 .1 1 0 .120.06 0.06 0.061.22 1 .33 1 .32o.24 0.34 0.33

34 52 46.78 45 440 60 1.00 0.96

14.35 3.52 4.70

0.54 0 13 0 .18100 37 9974 99.84

1.984 1.983 1 9830.016 0 .017 0 .017

2.000 2.000 2.0000 025 0.006 0.0080.004 0.004 0.0040.002 0.002 0.0020.025 0.01 2 0.0140.075 0 086 0.0840.007 0.011 0.0101 j72 1 .676 1 .6180.021 0.037 0.0350.637 0.163 0.2160.000 0.001 0.0010.043 0.01 1 0.0152.011 2.009 2.007

0 052 0.0330.722 0 7 44

0 0392.029

21.132.846.1

1.6 44.9 43.612I 30.4 30.885.6 24I 25.6

1 .183 1 .1910.017 0.01 60 .016 0 .019

0.016 0.0072.032 2.017

1 . 9 1 . 037 1 38.160 9 60.9

0.0512.036

3 1 . 8325

32.74 .1

63.2

12.0 15.14 . 3 4 2

83.7 80.7

Santanoni quadrangle most closely resemble exsolutiontextures in pyroxenes from unmetamorphosed igneousrocks.

Regardless of their 100Fe/(Fe + MC) ratios, all pyrox-enes that contain relict high-temperature lamellae havepartially broken down to intergrowths of augite and or-thopyroxene oriented along (100) ofthe host grain. Thisbreakdown suggests that regional metamorphism tookplace at temperatures below the stability of pigeonite. Thisconclusion is entirely consistent with metamorphicgeothermometry (Bohlen et al., 1985) and with pyroxenephase relations (Lindsley, 1983).

If initial exsolution and inversion took place duringcooling ofthese rocks from igneous conditions, as is con-sistent with the histories of pyroxenes from both the Nainand Laramie anorthosite massifs (Huntington, 1980; Frostand Lindsley, 1981; Ranson, 1986; Livi, 1987), then or-thopyroxene as well as pigeonite equilibria apply to theigneous part ofthese rocks'histories. The orthopyroxenehost in sample AL-4 (Fig. 6 and Table l) requires a min-imum pressure in excess of 4.6 kbar if inversion tookplace at approximately 800 'C, according to the geoba-rometer of Bohlen and Boettcher (1981). An ideal ionicmodel as described by Bohlen et al. (198 l) was used tocorrect for comoonents other than Ms and Mn. The min-

imum pressure of 4.6 kbar is a conservative estimate sinceit assumes that nonquadrilateral components are com-pletely partitioned into orthopyroxene, that inversion tookplace at a temperature 25 'C below the minimum stabilityofpigeonite (Lindsley, I 983), and that the orthopyroxenecontained 30/o CaSiO, at 800 'C. Bohlen and Boettcherhave suggested that this is the maximum CaSiO, com-ponent for Fe-rich orthopyroxene at temperatures belowapproximately 850 "C. A minimum crystallization pres-sure of approximately 6.3 kbar for sample AL-4 is a muchmore reasonable estimate according to Lindsley's (1983)forbidden-zone boundaries, but even ifone uses the con-servative pressure estimate based on orthopyroxene, theminimum pressure required for the crystallization of AL-4is well in excess of the 3-kbar maximum suggested byValley and O'Neil (1982) for anorthositic rocks at Wills-boro, New York. Likewise, the metamorphic pressure de-termined for sample PO-17 (approximately 8 kbar) byJafe et al. (1978) and Bohlen and Boettcher (1981) alsorepresents a minimum igneous pressure if inverted pi-geonite in this rock inverted as it cooled from igneousconditions.

The association of anorthosites with granitic and sy-enitic rocks is a worldwide phenomenon, and althoughgeochemical evidence has ruled out a comagmatic origin

2',10 OLLILA ET AL.: PYROXENE EXSOLUTION

H d

Fig. 8. Photomicrograph of inverted pigeonite from samplePO-17. Orthopyroxene host is at partial extinction, and (100)augite lamellae are vertical. This grain is the pigeonite equivalentof stage 6 in Fig. 2. No trace of original high-temperature la-mellae are visible, and augite lamellae have coalesced and coars-ened. This grain was chosen for optical reintegration because itis not in contact with an augite grain. Bulk compositlons weredetermined both including and not including the larger areas ofaugite. Both compositions are listed in Table I and are withinthe pigeonite range. Optical reintegration values were checkedby means of energy-dispersive analysis on a scanning electronmicroscope. Four areas that were as large as possible whileavoiding cracks were scanned. Results are plotted in Fig. 9.

for these rocks, it is commonly thought that they are co-genetic or at least roughly coeval (Emslie, 1978; Morse,l98l; Philpotts, 198 l; Mclelland, 1986). Furthermore,it has commonly been assumed in the Adirondacks thatthe gradational contacts between anorthositic and syenit-ic rocks (de Waard and Romey, 1969; Davis, 1971; Jaffeet al., 1983) imply that these rocks were magmas at thesame time and that mixing occurred between the tworock types. This sort of relationship has been documentedfor the Nain anorthosite massif by Wiebe (1980), and ifsuch is the case for the Adirondacks, then crystallizationpressures for pyroxene-bearing quartz syenites are appli-cable to anorthosite. Given the discrepancy, however, be-tween the conclusions Dresented here and the conclusions

o l a m e l l a e. hos t , f r om

Ja f fe e t a l . ( 1 978 )r op t i cE l l g re in teg rE ted

b u l k c o m p o s i t i o n

" SEM re in teg rE tedb u l k c o m p o s i t i o n

Fig. 9. Energy-dispersive analyses ofexsolution lamellae andinverted pigeonite in sample PO-17 plotted on the Fe-rich partof the pyroxene quadrilateral. Analyses were obtained by use ofa Kevex quantitative match routine using host compositions fromJaffe et al. (1978) as standards. Pyroxenes were analyzed for Ca,Mg, Fe, and Si and are plotted as molo/o Wo [Cal(Ca + Mg +Fe)1, Fs [Fe/(Ca + Ms + Fe)], and En [Mgl(Ca + Mg + Fe)].

of Valley and O'Neil (1982) and Valley (1985), the as-sumption that pyroxene-bearing quartz syenites intrudedat the same time as anorthosite may have to be re-ex-amined in more detail.

There is some isotopic evidence to support the conten-tion that pyroxene-bearing quartz syenites were intrudedat a different time than anorthosite. Silver (1969), on thebasis of a U-Pb study of zircons from Adirondack sy-enitic rocks, concluded that these rocks had an igneouscrystallization age of approximately 1130 Ma. In con-trast, Ashwal and Wooden (1983) concluded, on the basisof a Nd-Sm study, that anorthosite in the Mount Marcymassif in the Adirondacks has a crystallization age ofapproximately 1288 Ma. Critical evaluation of these ages,based upon different decay systems, must await more data,but it at least appears possible that anorthositic and sy-enitic rocks were intruded at different times and underdifferent conditions. A better understanding of the rela-tionship between anorthositic and syenitic rocks awaitsmore detailed isotopic studies, a more complete under-standing of the wollastonite deposit at Willsboro, NewYork, and an understanding of the relationship betweenthe anorthosite found at Willsboro and that found in theMarcy massif. If, however, conclusions regarding shallowemplacement of anorthosite and the deep emplacementof syenitic rocks are both correct, then a model such as

tF s

I O kbarm o l e 5

2es-r roo:10:o:g

OLLILA ET AL.: PYROXENE EXSOLUTION

En 25 50 75 tat

Fig. 10. Compositions ofpyroxenes from anorthositic and syenitic rocks from the Adirondacks and Labrador (PL-187) plotted

on the 10-kbar solvus ofLindsley (1983). P-10 is from Bohlen and Essene (1978) and PL-187 is from Ranson (1986). The dashed

line is the l0-kbar boundary ofthe "forbidden zone" (Lindsley, 1983). Pyroxene compositions to the right ofthis line are metastable

with respect to the assemblage augite + olivine + q:uartz. The 5-kbar "forbidden-zone" boundary (dotted line) is also shown.

Reintegrated compositions of exsolved grains are plotted except for sample AL-4. Alteration made reintegration of inverted pigeonite

impossible in sample AL-4; however, analyses of host augite and orthopyroxene were obtained. Exsolution textures (Fig. 8) clearly

indicate that this sample contains inverted pigeonite. Sample PO- I 7 suggests that crystallization of these rocks took place at pressures

greater than or equal to approximately 9 kbar.

presented by Martignole (1986) in which anorthosite in-trusion is followed almost immediately by crustal thick-ening and in which syenites are intruded into thickenedcrust provides at least one solution to this problem.

CoNcr,usroNs

The complex exsolution textures described above areinterpreted to result from the combined effects of coolingfrom igneous conditions and regional metamorphism.Adirondack pyroxenes differ from typical igneous pyrox-enes in that relict coarse exsolution lamellae have brokendown to intergrowths of augite and orthopyroxene ori-ented along (100) of the host grain. Reintegrated com-positions of magnesian to moderately Fe-rich pyroxenes(samples 9-17, GB-2, and BA-5) showing relict igneousexsolution textures record temperatures well above re-gional metamorphic temperatures and confirm that theseare igneous rather than metamorphic pyroxenes. We in-terpret the mobilization of exsolution lamellae to becaused by regional metamorphism at temperatures below

271

E n

-J

Fig. I l. Schematic phase relations among pyroxenes, and o1-ivine * quartz (ol) at different temperatures (Tl < T2 < T3)and constant pressure. The three-phase triangle opx-aug-ol (Zl)moves to the left, and the Ca content of opx increases as tem-perature increases. At 72, pigeonite becomes stable and formsvia the reaction augite + orthopyroxene * olivine + qtf,artz:pigeonite. As temperature further increases, the three-phase tri-angle pig-aug-ol moves to the left, and the gap in Ca contentbetween pigeonite and augite narrows (73). These trends areevident in the boundaries ofthe "forbidden zones" in the solviofLindsley (1983) and Fig. l0 ofthis paper. Increased Ca con-

tent stabilizes both orthopyroxene and pigeonite relative to oliv-ine + quartz when the assemblage is not saturated with respect

to augite. If the assemblage is saturated with augite, then in-

creased Ca content of either orthopyroxene or pigeonite reflects

an increase in temperature that in turn stabilizes olivine + quartz

relative to low-Ca pyroxene.

272 OLLILA ET AL.: PYROXENE EXSOLUTION

those necessary to stabilize pigeonite during the Grenvil-lian metamorphic event. This conclusion is based on thedifferences between Adirondack pyroxenes and typical ig-neous pyroxenes, the ubiquitous occurrence of (100) in-tergrowths ofaugite and orthopyroxene regardless oftheFe/Mg ratio of the pyroxene, and the correlation betweenthe degree ofpreservation ofrelict igneous exsolution tex-tures and metamorphic fabric in the host rock.

Fe-rich inverted pigeonite from sample AL-4 containscoarse "001" augite lamellae that are most reasonablyinterpreted as resulting from cooling from igneous con-ditions, and these lamellae have partially broken downto intergrowths of augite and orthopyroxene. Low-Ca py-roxene from sample PO-I7 has a bulk composition thatclearly indicates original crystallization as pigeonite eventhough low-Ca pyroxenes in this rock now consist ofin-tergrowths of orthopyroxene and augite with augite la-mellae oriented along (100) ofthe host. The compositionsof these pyroxenes, as well as pyroxenes described byBohlen and Essene (1978), require crystallization pres-sures well in excess of 3 kbar, and the composition oflow-Ca pyroxene in sample PO-17 (Fig. l0 and Table l)suggests a minimum igneous crystallization pressure ofgreater than approximately 9 kbar. Fe-rich igneous py-roxenes indicate that models requiring shallow emplace-ment of Adirondack anorthosite cannot be applied to thesyenitic rocks that surround the Marcy anorthosite mas-sif.

AcxNowr,BlcMENTS

This paper, in part, is derived from Paul Ollila's Ph.D research at theUniversity of Massachusetts The authors would like to thank DavidLeonard of the University of Massachuselts and Jerry Calvin of VassarCollege for their assistance with electron microprobe and sprra-roa anal-ysis An early version of this manuscript was reviewed by William Ran-son, and his comments and suggestions were most helpful. A critical re-view by Jacques Martignole also significantly improved the quality ofthispaper.

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M.a.Nuscnrpr RECETvED Ocrosen 13, 1986M.nNuscprm AccEmED Noveunen 20. 1987

AppnNorx 1. S,tnnpm DESCRIPTIoNS

9-17. Coarse-grained subophitic gabbroic anorthosite consist-ing of plagioclase (An46) 800/0, augite I 6010, inverted pigeonite I 0/0,

and ilmenite 30/0. Sample was collected near the summit of Don-aldson Mountain, Santanoni quadrangle.

AI-4. Medium- to coarse-grained, pyroxene-plagioclase-quartz-microperthite gneiss consisting of qttartz 160/0, microperthite 610/0,oligoclase 150/0, augite 2.50/0, inverted pigeonite l.4o/o, hom-blende 3.00/o, gamet0.3o/o, opaques 0.30/0, apatite O.4o/o, and zir-con 0.10/0. Sample was collected in the northeastern corner ofthe Santanoni quadrangle on the top of a hill I km south ofCameras Pond.

BA-5. Fine-grained pyroxene monzonite granulite consistingof microperthite 190/o, plagioclase 25010, inverted pigeonite 130/0,avgite l4o/0, garnet 2jo/o, apalite 4o/0, and opaques 50/0. Mediumto coarse grains ofplagioclase and pyroxene, which retain relictigneous textures, are enclosed in a fine-grained granoblastic ma-trix consisting of microperthite, plagioclase, and garnet. Thesample was collected on Basin Mountain in the Mount Marcyquadrangle.

GB-2. Medium- to fine-grained gabbro ganulite consisting ofplagioclase (Anru to Anrr) 33ol0, orthopyroxene (includes invertedpigeonite) 150/0, augite l9o/0, gamet 160/0, ilmenite 110/0, magne-tite 2o/o, and apatite 4010. Pyroxenes occur both as granoblastic,fine-grained aggregates and as coarser (up to 2 mm) grains thatretain relict igneous exsolution textures. The sample was col-lected in Guideboard Brook in the Mount Marcy quadrangle.

OLLILA ET AL.: PYROXENE EXSOLUTION